CrystEngCommwww.rsc.org/crystengcomm Volume 15 | Number 35 | 21 September 2013 | Pages 6881–7130

COVER ARTICLEEchegoyen et al. Synthesis of a phthalocyanine and porphyrin 2D covalent organic framework www.chemspider.com

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Cite this: CrystEngComm, 2013, 15, 6892

Received 22nd April 2013,Accepted 28th May 2013

Synthesis of a phthalocyanine and porphyrin 2Dcovalent organic framework3

DOI: 10.1039/c3ce40706c

www.rsc.org/crystengcomm

Venkata S. Pavan K. Neti,a Xiaofei Wu,b Shuguang Dengb and Luis Echegoyen*a

A new phthalocyanine and porphyrin based hybrid covalent

organic framework, CoPc-PorDBA COF, has been synthesized and

its gas storage properties have been measured. The CoPc-PorDBA

COF crystallizes into 2D sheets with an eclipsed AA stacking motif,

and it showed high surface area (SBET = 1315 m2 g21), and can

store up to 0.8 wt% of H2 at 77 K and 1 bar and 0.6 wt% of CH4 at

298 K and 1 bar.

Phthalocyanine and porphyrin covalent organic frameworks1

(COFs) have attracted considerable attention due to theirphotocurrent generation properties,2 applications in optoelectro-nic devices3 and their potential in organic photovoltaics (OPVs). Inaddition to these attractive applications, some 2D and 3D covalentorganic frameworks4 and metal organic frameworks5 (MOFs),initially designed for gas storage applications, have achieved highstorage capacities of H2 and CH4 at 77 K and 298 K within apressure range of 0–100 bar. Preparation of COFs with highsurface area with adjustable pore size through the rational designand incorporation of good electron donors into a framework toimprove light absorption is a challenging problem.Phthalocyanines and porphyrins are highly stable and intenselycolored aromatic macrocyclic compounds and can be found inmany coordination complexes,6 and luminescent devices.7

Recently, several metallophthalocyanine COFs have been reportedwith Zn, Ni, and Co as central metal ions.1,2 To date, the H2 andCH4 storage capacities of a phthalocyanine and porphyrin COFwith a high surface area has not been reported. Here we report anew boronate ester based COF that exhibits a H2 storage capacityof 0.8 wt% at 77 K, 1 bar and 0.6 wt% CH4 at 298 K, 1 bar. ThisCOF contains two very good electron donors which absorb indifferent regions of the UV-Vis spectra in a single framework. TheCOF is based on octahydroxy phthalocyanine Co(II) (1) and 5,15porphyrin diboronic acid (2) (Scheme 1). The chemical connectiv-

ity and composition of the CoPc-PorDBA COF were determined byseveral methods such as Powder X-ray Diffraction (PXRD), infraredspectroscopy, solid-state 11B and 13C CP-MAS NMR, surface areameasurements and elemental analysis. The CoPc-PorDBA COFwas synthesized under solvothermal conditions in a mixture ofdimethylacetamide and o-dichlorobenzene (2/1 v/v) in a sealedglass ampoule at 120 uC for 72 h (see ESI3).

The CoPc-PorDBA COF was isolated in 65% yield as a greenpowder that is totally insoluble in common organic solvents such

aDepartment of Chemistry, University of Texas at El Paso, El Paso, TX 79968, United

States. E-mail: echegoyen@utep.edu; Fax: +1 915-747-8807; Tel: +1 915-747-7573bDepartment of Chemical Engineering, New Mexico State University, Las Cruces, NM

88003, United States

3 Electronic supplementary information (ESI) available: Experimental details, Fig.S1–S8. See DOI: 10.1039/c3ce40706c

Scheme 1 Synthesis of CoPc-PorDBA COF from 5,15 porphyrin diboronic acidand (OH)8Pc Co(II).

CrystEngComm

COMMUNICATION

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as dimethylsulfoxide, dichloromethane, N,N-dimethylformamide,tetrahydrofuran etc. The solid state 13C with (CP-MAS) NMRspectrum (Fig. S3, ESI3) of the CoPc-PorDBA COF shows ninesignals at d = 173.44, 157.86, 143.95, 134.64, 127.72, 115.23, 106.06,100.94 and 97.11 ppm, which were assigned to the carbon atomsof the phthalocyanine and phenyl groups by comparison to thespectrum of octahydroxy phthalocyanine Co(II) (Fig. S4, ESI3). The11B magic angle spinning (MAS) NMR spectrum of the CoPc-PorDBA COF display similar features to those of other reportedCOFs and to those of the starting materials.2a PXRD (Cu Karadiation) was used to determine the crystallinity of thesynthesized CoPc-PorDBA COF. The CoPc-PorDBA COF displaysan intense diffraction peak at a 2h angle (d-spacing) of 2.63u(36.14 Å), less intense peaks at 3.52u (25.55 Å), 5.31 (18.07 Å), 7.74u(7.33 Å), and a broad peak at 25.3 (24.54 Å) (Fig. 1). This patternwas manually indexed into a primitive tetragonal unit cell with cellparameters a = 36.14 Å and c = 3.62 Å similar to other previouslyreported primitive tetragonal NiPc and ZnPc COFs.1,2 Crystalmodels were generated using the Materials Studio modeling suite,using eclipsed sheets of CoPc-PorDBA COF (Fig. 1 inset) in a sqltopology8 in the space group P4/mmm, obtaining simulated cellparameters of a = 36.14 Å and c = 3.62 Å, which match well withthe indexed values. Simulated PXRD pattern matched well themeasured one (Fig. 1). Pawley refinement was subsequentlyperformed to refine the unit cell parameters and to confirm theassignments of the observed diffraction peaks, obtaining values ofa = 36.54 Å and c = 3.67 Å with good residuals (see ESI3).

In addition to PXRD, the Fourier-Transformed Infraredspectrum showed a stretching frequency at 1344 cm21 which

corresponds to the newly formed boronate ester linkage between 1and 2 (see ESI3) including attenuation of peaks at 3400 cm21 and3140 cm21, which correspond to hydroxyl groups of thephthalocyanine and porphyrin diboronic acid, respectively. Thelayered structure of the COF was also observed by Field-emissionscanning electron microscopy (FE-SEM). The SEM image revealedits layered sheet structure (Fig. 2) and flake-like morphology. Toevaluate the porosity of the CoPc-PorDBA COF, N2 adsorptionisotherms were measured at 77 K. A Brunauer–Emmet–Teller(BET) surface area of 1315 m2 g21 was obtained with a type IVisotherm typical of mesoporous materials and with reversiblesorption profiles (Fig. 3). The total pore volume was calculated tobe 0.88 cm3 g21 and the average pore size was calculated usingnonlocal density functional theory (NLDFT), and found to beapproximately 3.5 nm (see ESI3).

Once the permanent porosity of CoPc-PorDBA COF wasestablished, we evaluated its potential for H2 and CH4 storage.The hydrogen storage capacity of CoPc-PorDBA COF is 0.8 wt% at77 K, 1 bar (Fig. 4a) which is comparable to other crystalline 2DCOFs such as COF-102 (0.8 wt% at 1 bar). In direct contrast to theH2 uptake, the CH4 isotherm for CoPc-PorDBA revealed an uptakeof only 0.6 wt% at 298 K, 1 bar (Fig. 4b). We believe that the loweruptake of methane compared to hydrogen may be due to the sizedifference and to the absence of metal–hydrogen interactions.

The UV-Vis diffuse reflectance powder spectra of CoPc-PorDBACOF (Fig. 5, ESI3) showed strong absorption bands at 220, 252,282, 453, 590, 625 and 730 nm and CoPc showed absorption peaks

Fig. 1 XRD pattern of CoPc-PorDBA COF with the observed pattern in black,Pawley refined profile in red, and the difference plot in black (observed minusrefined profiles). The calculated XRD pattern from the proposed models isshown in blue. Inset: eclipsed stacking representation of CoPc-PorDBA COFbased on powder diffraction and modeling projected along the c axis (H atomsare omitted). C, B, and O are represented in turn as gray, purple, and redspheres.

Fig. 2 Scanning electron microscope (SEM) image of the CoPc-PorDBA COFgrown under solvothermal conditions.

Fig. 3 Nitrogen adsorption (filled symbols) and desorption (empty symbols)isotherm curve and surface area.

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at 280, 595, 650 and 735 nm. The CoPc electronic absorptionspectrum (Fig. S5, ESI3) in dilute dichloromethane solutionshowed strong absorption bands at 295, 342, 443, 615 and 740nm which are typical of metallophthalocyanines. As explained inother reports1a the blue shift of solid state CoPc-PorDBA (35 nm)of the B band when compared to that for CoPc(OMe)8 indichloromethane solution is due to the difference in aggregatedphthalocyanines in the solution and in the solid state (liquidcrystalline) while the red shift of the Q band is due to thedifferences in aggregation geometry as well as the electron-withdrawing nature of the boronate esters. The PorDBA solid-statediffuse reflectance spectrum (Fig. S6, ESI3) shows a low intensityband at about 415 nm, typical for the Soret band of the UV-Visabsorption spectra of porphyrins in homogeneous solutions (Fig.S5, ESI3). The low intensity of the Soret band may be due tostacking instead of aggregation of the porphyrins in the COF. TheCoPc-PorDBA COF showed some similarities when compared toindividual electronic absorption spectra of CoPc(OMe)8 andPorDBA. Thermogravimetric analysis of the CoPc-PorDBA COFshowed high thermal stability (see ESI3), retaining 90% of its massat 300 uC.

Conclusions

In conclusion, we have synthesized and characterized a new cobaltbased phthalocyanine and porphyrin covalent organic frameworkwith a high surface area and reasonable H2 and CH4 storagecapacities. We connected two very good electron donors whichabsorb in different regions of the UV-Vis spectrum. This COF ispotentially useful in photovoltaic applications and is currentlybeing evaluated as such.

Acknowledgements

This work was generously supported by NSF grant CHE-1110967, and the Robert A. Welch Foundation, grant # AH-0033. We are very thankful to Professor William R. Dichtel(Cornell University) for providing us the powder X-raydiffraction simulation of the CoPc-PorDBA COF.

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Fig. 4 (a) Hydrogen and (b) methane adsorption–desorption isotherm curves ofCoPc-PorDBA COF.

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